Fuel cells (FCs) have high power generation efficiency and impose less burden on the environment, and thus they are expected to be widely used as a distributed energy system in the future. Particularly, polymer electrolyte fuel cells that use a cation (hydrogen ion) conductive polymer electrolyte are expected to be utilized in mobile units such as automobiles, distributed power generation systems, home cogeneration systems, and so on because they have high output density, can operate at low temperatures, and can be made smaller.
Polymer electrolyte fuel cells generate electricity and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. FIG. 17 is a schematic cross-sectional view illustrating an example of the basic structure of a unit cell designed to be installed in a conventional polymer electrolyte fuel cell. FIG. 18 is a schematic cross-sectional view illustrating an example of the basic structure of a membrane-electrode assembly (MEA) designed to be installed in the unit cell 100 shown in FIG. 17. FIG. 19 is a schematic cross-sectional view illustrating an example of a catalyst-coated membrane (CCM) that constitutes the membrane-electrode assembly 101 shown in FIG. 18.
As shown in FIG. 19, in the catalyst-coated membrane 102, a catalyst layer 112 composed of a hydrogen ion conductive polymer electrolyte and a catalyst-carrying carbon obtained by causing a carbon powder to carry an electrode catalyst (e.g. platinum metal catalyst) is formed on each surface of a polymer electrolyte membrane 111 that selectively transports hydrogen ions. As the polymer electrolyte membrane 111, polymer electrolyte membranes made of perfluorocarbon sulfonic acid (e.g., Nafion (trade name) available from E.I. Du Pont de Nemours & Co. Inc., USA) are widely used.
As shown in FIG. 18, the membrane-electrode assembly 101 is formed by forming a gas diffusion layer 113 having gas permeability and electron conductivity using, for example, carbon paper treated for water repellency on the outer surfaces of the catalyst layers 112. The combination of a catalyst layer 112 and a gas diffusion layer 113 forms an electrode 114 (anode or cathode). As shown in FIG. 17, the unit cell 100 is formed of the membrane-electrode assembly 101, gaskets 115 and a pair of separators 116. The gaskets 115 are disposed on the outer periphery of the electrodes with the polymer electrolyte membrane interposed therebetween so as to prevent the supplied fuel gas and oxidant gas from leaking out and to prevent them from mixing with each other. The gaskets 115 are usually combined in advance with the electrodes and the polymer electrolyte membrane. The combination of the polymer electrolyte membrane 111, a pair of electrodes 114 (each including a catalyst layer 112 and a gas diffusion layer 113) and the gaskets 115 may also be called a “membrane-electrode assembly”.
A pair of separators 116 for mechanically fixing the membrane-electrode assembly 101 are disposed on the outer surfaces of the membrane-electrode assembly 101. In a portion of the separator 116 that contacts the membrane-electrode assembly 101, a gas flow path 117 for supplying a reaction gas (a fuel gas or oxidant gas) to the electrode and removing a gas containing an electrode reaction product and unreacted reaction gas from the reaction site to the outside of the electrode is formed. The gas flow path 117 can be formed independently of the separator 116, but is usually formed by providing a groove on a surface of the separator. On the other side of the separator 116 that is opposite from the membrane-electrode assembly 101, a cooling water flow path 118 is formed by providing a groove.
As described above, a single unit cell formed by fixing the membrane-electrode assembly 101 with a pair of separators 116 can produce an electromotive force of about 0.7 to 0.8 V at a practical current density of several tens to several hundreds mA/cm2 by supplying a fuel gas to the gas flow path of one separator and an oxidant gas is supplied to the gas flow path of the other separator. However, polymer electrolyte fuel cells are usually required to produce a voltage of several to several hundreds volts when used as a power source. For this reason, the required number of unit cells are connected in series to form a stack capable of providing the required voltage for practical use.
In order to supply reaction gases to the gas flow paths 117, there is required a manifold in which a pipe for supplying a reaction gas is branched into a number corresponding to the number of separators and the branched ends are directly connected to the gas flow paths on the separators. Particularly, a manifold in which an external pipe for supplying a reaction gas is directly connected to the separators is called an “external manifold”. There is another type of manifold called an “internal manifold”, which has a simpler structure. An internal manifold is composed of apertures provided in the separators in which a gas flow path is formed. The inlet and outlet are connected with the apertures. The reaction gas is supplied to the gas flow path directly from the aperture.
The gas diffusion layer 113 has the following three functions. The first function is to diffuse a reaction gas so as to supply the reaction gas uniformly from the gas flow path of the separator 116 that is located on the outer surface of the gas diffusion layer 113 to the electrode catalyst in the catalyst layer 112. The second function is to rapidly exhaust water produced by a reaction in the catalyst layer 112 to the gas flow path. The third function is to transfer electrons required for a reaction or produced electrons. Therefore, the gas diffusion layer 113 is required to have high reaction gas permeability, high water drainage capability and high electron conductivity.
Generally, in order to impart gas permeability to a gas diffusion layer 113, a porous conductive substrate that is produced using a carbon fine powder, pore-forming material, carbon paper, carbon cloth or the like is used. In order to impart water drainage capability, a water repellent polymer as typified by fluorocarbon resin or the like is dispersed in the gas diffusion layer 113. In order to impart electron conductivity, the gas diffusion layer 113 is formed using an electron conductive material such as carbon fiber, metal fiber or carbon fine powder. The surface of the gas diffusion layer 113 that contacts the catalyst layer 112 may be provided with a water repellent carbon layer made of a water repellent polymer and a carbon powder.
The catalyst layer 112 has the following four functions. The first function is to supply a reaction gas supplied from the gas diffusion layer 113 to the reaction site in the catalyst layer 112. The second function is to transfer hydrogen ions required for a reaction on the electrode catalyst or generated hydrogen ions. The third function is to transfer electrons required for the reaction or produced electrons. The fourth function is to accelerate the electrode reaction by its high catalytic performance and its large reaction area. Therefore, the catalyst layer 112 is required to have high reaction gas permeability, hydrogen ion conductivity, electron conductivity and catalytic performance.
Generally, in order to impart gas permeability to a catalyst layer 112, a catalyst layer having a porous structure and a gas channel is formed using a carbon fine powder or pore-forming material. Further, in order to impart hydrogen ion permeability, a hydrogen ion network is formed by dispersing a polymer electrolyte in the vicinity of the electrode catalyst of the catalyst layer 112. In order to impart electron conductivity, an electron channel is formed using, as a carrier for carrying the electrode catalyst, an electron conductive material such as a carbon fine powder or carbon fiber. In order to improve catalytic performance, a catalyst element composed of a very fine particulate electrode catalyst with a particle size of several nm that is carried on a carbon fine powder is dispersed at a high density in the catalyst layer 112.
For the commercialization of polymer electrolyte fuel cells, various attempts have been made to improve the performance of catalyst layer 112. For example, to increase the utilization rate of catalyst in the catalyst layer, Patent Document 1 has proposed to adjust the catalyst distribution ratio between the cathode and the anode according to the gas concentration ratio of the anode and the cathode. Specifically, the catalyst distribution ratio of the cathode catalyst layer is increased in a region in which the gas concentration is higher at the cathode side than the anode side, whereas the catalyst distribution ratio of the anode catalyst layer is increased in a region in which the gas concentration is lower at the cathode side than the anode side. In other words, an electrode in which the catalyst distribution ratio is changed in the plane direction of a catalyst layer has been proposed.
A unit cell for a fuel cell, in which the composition of constituent materials is changed in the plane direction of a catalyst layer, has been disclosed in Patent Document 2. Specifically, a method has been proposed in which, in the cathode catalyst layer, the amount of catalyst is increased near the oxidant gas outlet than near the oxidant gas inlet, so as to prevent an excessive amount of produced water from staying in the cathode electrode.    Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 7-169471    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 8-167416